The present invention relates to suspension insulators having excellent tensile strength and particularly to the form of an embedding multi-step portion of a metal pin.
The suspension insulator is usually one wherein cap hardware is provided at an insulating body head and an embedding expanded portion at an upper end of a metal pin is embedded in and secured to a pin assembling hole provided in the insulating body with cement. It has been known that the tensile strength of such a suspension insulator is greatly influenced by the shape of the embedding portion positioned in the pin assembling hole. When a tensile load is applied to the suspension insulator, a compression force acts on a side wall of a ceramic insulating body head owing to a wedge effect of the cap hardware and the metal pin, and a stress is distributed on the side wall of said head portion owing to this compression force. When the maximum tensile stress value exceeds the specific tensile stress of the head portion of the ceramic insulating body, the insulating body is broken. Since, in the suspension insulator, the maximum tensile stress point is located near a boundary portion between the upper surface and the inside surface of the pin assembling hole provided in the insulating body in view of the mechanism, the shape of the embedding portion at the upper end of the metal pin has an important role in influencing the tensile strength. In prior metal pins of such a suspension insulator, a metal pin which has a spherical convex trapezoid expanded portion 11 (hereinafter called "R-shaped") as shown in FIG. 6, A, and a tapered metal pin which is an inverted frusto-conical expanded portion 12 as shown in FIG. 6, B, have been usually adopted as the shape of the embedding portion provided at the top end of the metal pin. In the former, the stress concentration to the insulating body is reduced by the spherical convex portion, so that aging deterioration of the insulating body can be prevented; but in order to develop a satisfactory wedge effect, the expanded portion should have a fairly large size and this is not preferable in view of making the entire suspension insulator of small size and light weight. On the other hand, the latter develops a satisfactory wedge effect but the insulating body readily causes aging deterioration due to the stress concentration.